DTIC ADA268011: Opposing Mesoscale Flows in a Broken Midlatitude Squall Line

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by  Defense Technical Information Center

AD-A268 011
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Spring 1993 THESIS
Opposing Mesocale Flows in a Broken Midlatitude Squall
line
Capt Barbara D. Minor
AFIT Student Attending: Colorado State University AFIT/CI/CIA- 93-012
AFIT/CI
Wright-Patterson AFB OH 45433-6583
Approved for Public Release lAW 190-1
Distribution Unlimited
MICHAEL M. BRICKER, SMSgt, USAF
Chief Administration DTIC
AUG 17 19
AP
93-19049
115
DISCLAIMER NOTICE '
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QUALITY AVAILABLE. THE COPY
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THESIS
OPPOSING MESOSCALE FLOWS IN A BROKEN MIDLATITUDE SQUALL LINE
Submitted by
Barbara D. Miner
Department of Atmospheric Science
In partial fulfillment of the requirements
for the degree of Master of Science
Colorado State University
Fort Collins, Colorado
Spring 1993
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COLORADO STATE UNIVERSITY
December 14, 1992
WE HEREBY RECOMMEND THAT THE THESIS PREPARED UNDER OUR
SUPERVISION BY BARBARA D. MINER ENTITLED OPPOSING MESOSCALE
FLOWS IN A BROKEN MIDLATITUDE SQUALL LINE BE ACCEPTED AS FUL-
FILLING IN PART REQUIREMENTS FOR THE DEGREE OF MASTER OF SCI-
ENCE.
Committee on Graduate Work
Committee Member
Committce (cid:127),lcliber D ,!G QUALITlY '143P-ECYI'ED 3
Depairtment Heat 1'
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ABSTRACT OF THESIS
OPPOSING MESOSCALE FLOWS IN A BROKEN MIDLATITUDE SQUALL LINE
During the period 14-15 June 1985 a broken line of convection with one primary gap
(echo-free) region developed along a cold front passing through the Oklahoma-Kansas Pre-
liminary Regional Experiment for STORM-Central (OK PRE-STORM) domain. Radar
and satellite data are presented to provide an overview of the life cycle of the line. Ob-
servations from the OK PRE-STORM mesonetwork and upper air soundings are used to
document the occurrence of the gap and an associated surface mesolow.
The convective line initially developed as two mesoscale convective systems (MCSs),
one in northeast Kansas, the other in the Texas panhandle, along a weak cold front. As
the two MCSs matured, convection developed between the two similar to the broken-line
squall line formation described by Bluestein and Jain (1985). Despite strong low-level
convergence and strong moisture convergence, an echo-free region remained between the
two MCSs throughout the life cycle of the line.
The upper level flow pattern along the line of convection showed strong upper level
outflow from the two MCSs converging over the echo-free region and strong subsidence in
that region from 250 mb to 650 mb. It is hypothesized that the strong mid- and upper
level subsidence was the main factor in the lack of convection in the echo-free region.
During the mature phase of the line, a surface mesolow developed within the echo-
free region. Calculations are made using a form of the hypsometric equation to determine
if the presence of the surface mesolow could have been produced by the mid and upper
level subsidence found in that region. At 0300 UTC the mesolow was 2 mb lower than
the surrounding areas. Calculations show that subsidence warming in the column could
account for a drop in pressure of .75 mb.
nii.
The results of the study show that while strong low-level convergence existed all along
the front throughout most of its life cycle, mid- and upper level outflow from the existing
MCSs prevented convection in the echo-free region. The resulting subsidence contributed
to the formation of the surface mesolow in the echo-free region. This study shows the need
for the evaluation of upper-level forcing mechanisms when forecasting the development of
thunderstorms along fronts and convergence zones.
Barbara D. Miner
Department of Atmospheric Science
Colorado State University
Fort Collins, Colorado 80523
Spring 1993
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ACKNOWLEDGEMENTS
I would like to express my deepest appreciation to my advisor, Dr. Richard Johnson,
for his guidance and dedication to the successful, timely completion of this work. I would
also like to thank my other committee members, Dr. Thomas McKee and Dr. Bogusz
Bienkiewicz.
I would also like to thank Paul Ciesielski, Rick Taft, Bill Gallus, Bob Falvey, Xin Lin,
and Scot Loehrer for their willingness to share their programming skills and help with the
use of the computer systems at the Atmospheric Science Department.
Thanks are also extended to Jos6 Meitin for providing the mesonetwork and upper
air data, as well as his help acquiring, in conjunction with Robert Hueftle, the radar
reflectivity composites.
Finally, thanks to Gail Cordova, who was instrumental in the completion of this
manuscript and Judy Sorbie-Dunn, who drafted several of the figures.
This research was supported by the National Science Foundation Grant No. ATM-
9013112.
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TABLE OF CONTE.4TS
1 INTRODUCTION 1
2 BACKGROUND 3
2.1 Organization of mesoscale convective systems ................... 3
2.2 M esoscale circulations ................................ 8
2.3 Subsidence warming and the mesolow ....................... 12
2.4 Interactions between mesoscale convective systems ...................... 13
3 DATA SET AND ANALYSIS PROCEDURES 15
3.1 PRE-STORM ..................................... 15
3.2 Surface data . .. . . ... .. . .. . . .. .. . .. . .. .. . . . . .. . .. . . 15
3.2.1 Pressure adjustments ............................... 17
3.3 Upper air data ........ ............ ....... ........ . 19
3.3.1 Data network and adjustments to upper air data ...................... 19
3.3.2 Calculated fields .......... .................................. 19
3.4 Radar Data and Satellite Imagery ........ ......................... 21
4 SYNOPTIC OVERVIEW 22
4.1 Synoptic conditions ......... ................................. 22
4.2 Satellite and radar overview ........ ............................. 30
5 MESOSCALE ANALYSIS 39
5.1 Surface observations ......... ................................. 39
5.2 Upper Air Analyses ......... ................................. 47
5.2.1 2100 UTC .......... ...................................... 53
5.2.2 0000 UTC .......... ...................................... 53
5.2.3 0300 UTC ........... ...................................... 69
5.2.4 0600 UTC .......... ...................................... 77
5.3 Surface an6 Upper Air Synopsis ........ .......................... 77
6 MESOSCALE ANALYSIS OF ECHO-FREE REGION AND
MESOLOW 82
6.1 Development of the echo-free region ........................ 82
6.1.1 2100 UT C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
6.1.2 0000 UT C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
6.1.3 0300 UTC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
6.1.4 0600 UT C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
6.2 Formation of the surface mesolow in the echo-free region ............. 96
6.3 Schematic ............................................... 100
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7 SUMMARY 105
REFERENCES 107
A INSTRUMENT BIAS CORRECTIONS 114
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LIST OF FIGURES
2.1 Schematics depicting (a) symmetric and (b) asymmetric types of precipitation
patterns (from Houze et al., 1990) ............................... 6
2.2 Schematics depicting (a) symmetric and (b) asymmetric types of precipitation
patterns with pressure field overlaid (from Luehrer, 1992) ............... 7
2.3 Schematic showing idealized depiction of squall line formation (from Blues~ein
and Jain, 1985) .......... ................................. 9
2.4 Conceptual model of a squall line with a trailing stratiform region viewed per-
pendicular to the convective line (from Houze et al., 1989) ............. 10
2.5 Schematic of hypothesized mesoscale circuiations for two opposing mesoscale
convective systems. (from Stensrud and Maddox, 1988) ............... 14
3.1 The PRE-STORM observational mesonetwork (from Meitin and Cunning, 1985). 16
3.2 The PRE-STORM Portable Automated Mesonetwork(PAM) and Surface Au-
tomated Mesonetwork (SAM) surface array ........................ 18
3.3 The PRE-STORM sounding mesonetwork ............................ 20
4.1 1200 UTC 14 June 1985 surface and upper air analyses: (a) surface; (b) 850 mb;
(c) 700 mb; (d) 500 mb; (e) 300 mb ............................. 23
4.1 continued .......... ....................................... 24
4.1 continued .......... ....................................... 25
4.2 Same as 4.1 except at 0000 UTC, 15 June 1985 ....................... 27
4.2 continued .......... ....................................... 28
4.2 continued .......... ....................................... 29
4.3 15 June 1985 infrared (IR) satellite imagery for: (a) 0000 UTC; (b) 0130 UTC;
(c) 0200 UTC; (d) 0300 UTC; (e) 0500 UTC; (f) 0600 UTC; (g) 0700 UTC;
(h) 0800 UTC; and (i) 0900 UTC ....... ........................ 31
4.3 continued .......... ....................................... 32
4.3 continued .......... ....................................... 33
4.4 Radar composites of RADAP-II digitized data for 15 June 1985 ............ 35
4.4 continued .......... ....................................... 36
4.4 continued .......... ....................................... 37
5.1 Surface analysis of adjusted pressure at 2100 UTC 14 June 1985 ........... 40
5.2 Surface analysis of adjusted pressure at 0000 UTC 15 June 1985 ........... 41
5.3 Same as Fig. 5.2 except for 0100 UTC 15 June 1985 ..................... 43
5.4 Same as Fig. 5.2 except for 0200 UTC 15 June 1985 ..................... 44
5.5 Same as Fig. 5.2 except for 0230 UTC 15 June 1985 ..................... 45
5.6 Same as Fig. 5.2 except for 0300 UTC 15 June 1985 ..................... 46
5.7 Same as Fig. 5.2 except for 0430 UTC 15 June 1985 ..................... 48
5.8 Same as Fig. 5.2 except for 0500 UTC 15 June 1985 .................... 49
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